Struvite kidney stones

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What are struvite kidney stones?
Struvite kidney stones are mostly composed of struvite (magnesium ammonium phosphate). Struvite calculi also contain deposits of calcium carbonate-apatite.

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Struvite kidney stones can form only when saturated levels of magnesium ammonium phosphate are present in urine. Magnesium ammonium phosphate solubility decreases with the increase in the urine pH. In alkaline urine, struvite crystals form readily and get deposited. Struvite stones account for 10-15% of renal calculi in USA and they can grow rapidly to form staghorn calculi.

Cause of struvite kidney stones

Magnesium ammonium phosphate calculus form when the ammonium, magnesium and phosphate ions are present at saturation levels and also the urine pH is high. In the first instance, ammonium phosphate (NH4+) has to be formed in the urine for these struvite stones to develop. In ureotelic organisms, including humans, the proteinaceous nitrogenous waste from the body is disposed off as urea in the urine. When there is upper urinary tract infection by certain urease-producing bacteria, urea is converted into ammonia.

Some of the urease-producing bacteria are, Proteus mirabilis, Staphylococcus species, Klebsiella species, Providencia species, Pseudomonas species, Corynebacterium species and Ureaplasma urealyticum. Struvite calculi are more common in women as they are more prone urinary infections.

Urease is a enzyme that catalyzes the hydrolysis of urea into carbon dioxide and ammonia. The reaction is as follows:
(NH₂)₂CO [urea] + H₂O —> CO₂ + 2NH₃ [ammonia]
The ammonia readily combines with water to form ammonium hydroxide.
NH₃ + H₂O —> NH4⁺ + OH^- —> NH₄OH.
Ammonium hydroxide is a weak alkali and it increases the pH of urine. It reacts with the magnesium and phosphate ions in the urine to form struvite.
6 H₂O + Mg²⁺ + NH4⁺ + PO4^3- —> MgNH₄PO₄6H₂O
When the pH of the urine is greater than the nucleation pH, there is rapid formation and growth of crystals.

Symptoms of struvite calculi presence

As there is concurrent infection along with struvite calculi development, fever and chills may be present. Other common symptoms are, nausea, loss of appetite, dull, burning, persistent pain, appearance of blood in the urine, cloudy urine and foul smelling urine. The calculi may grow rapidly to form staghorn calculi. The staghorn calculi may grow many branches and fill the renal pelvis. The branches may extend into the renal calyces.

In some cases struvite calculi may remain asymptomatic. An untreated struvite staghorn stone can damage the soft renal tissues, leading to end-stage pyelonephritis. The end-stage pyelonephritis may result in life-threatening sepsis and renal failure. As struvite stones are usually radiopaque, they can be detected by radiography or sonography.

Treatment and management of struvite calculus

Struvite calculus is always associated with the danger of rapidly developing into staghorn calculus.

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Struvite calculus must be removed completely, in the early course of the disease in order to prevent complications and renal deterioration. Conservative treatment of patients without removal of struvite stones had resulted in death of nearly 30% of the patients due to pyelonephritis, sepsis or renal failure.

With the calculus clearance, the patient must be treated with antibiotics to cure the renal infection. Persisting infection can cause recurrence of the struvite kidney stones. The recurrence rate is nearly 10%. Ineffective infection clearance, residual fragments or scar tissues had caused recurrence of struvite kidney stones in 80% of the patients.

In struvite calculi patients with significant medical comorbidities or acute infections wherein surgical procedures are contraindicated, nonsurgical measures are advised. Bacterial urease inhibitors such as, oral acetohydroxamic acid (AHA) are given. As AHA administration is associated with several adverse effects, the patients health status must be closely monitored.

Minimally invasive procedures like extracorporeal shockwave lithotripsy (SWL), flexible ureteroscopy and percutaneous nephrolithotomy (PNL) are resorted to remove the persisting struvite kidney stones. A percutaneous nephrostomy (PCN) tube may be required to allow adequate renal discharge.

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References:
1.http://kidney.niddk.nih.gov/kudiseases/pubs/stonesadults/
2.http://www.kidney.org/atoz/content/diet.cfm
3.http://kidney.niddk.nih.gov/kudiseases/pubs/kidneystonediet/index.htm
4.Kristensen C, Parks JH, Lindheimer M, Coe FL. Reduced glomerular filtration rate and hypercalciuria in primary struvite nephrolithiasis. Kidney Int 1987; 32:749.
5.Gnessin E, Mandeville JA, Handa SE, Lingeman JE. Changing composition of renal calculi in patients with musculoskeletal anomalies. J Endourol 2011; 25:1519.
6.Viprakasit DP, Sawyer MD, Herrell SD, Miller NL. Changing composition of staghorn calculi. J Urol 2011; 186:2285.

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